Heating devices



Sept. 15, 1910 T. B. JAWOR 3,529 1 HEATING DEVICES Filed June 5. 1968 3 Sheets-Sheet l J c M M 1 QZWWZL W W; [A16 fz0ZwQ Sept. 15, 1970 v JAWQR 3,529,115

HEATING DEVICES Filed June 3, 1968 5 Sheets-Sheet 2 m a (a l 51/ .5; [/11 Sepf. 15, 1910 v T. B. JAwoR 3,529,115

HEATING DEVICES Filed June 5, I968 s Sheets-Sheet 3 United States Patent US. Cl. 219-1055 11 Claims ABSTRACT OF THE DISCLOSURE A heating device, e.g. for preparing plastics material for moulding, utilises a beam of microwave radiation propagated through a wave-guide from a source at one end thereof, past an irradiating zone through which articles to be heated are conveyed, to the other end of the waveguide, and means are provided connecting said other end to an intermediate point of the wave-guide (between the source and the irradiating zone) to form a loop so that the beam passes any point around such loop at least twice.

This invention relates to heating devices in which the heating medium is microwave electromagnetic radiation.

It has previously been proposed to heat various goods or articles by placing them in a high-intensity microwave beam. For example, articles have been carried on a conveyor belt through a gap in a wave-guide, so that the articles pass successively through a microwave beam crossing the gap. Such an arrangement can be quite effective, but its efliciency is low especially with low loss materials as the beam must be of high intensity but much of the energy in the beam is not absorbed by any passing article and passes on through the waveguide where it goes to waste in a terminating load.

It is an object of the present invention to provide an improved heating device, employing microwave electromagnetic radiation as the heating medium, which device is capable of relatively high efficiency.

According to the invention, we provide a heating device comprising a source of microwave energy coupled to one end of a wave-guide so as to produce a beam of microwave electromagnetic radiation therealong, including means connecting the other end of said wave-guide to an intermediate point thereon so that the portion of the length of said wave-guide between said inter-mediate point and said other end is formed into a closed loop so that said beam passes any point around said loop at least twice, and means for conveying articles to be heated through an irradiating zone comprising a portion of the path of said beam within said loop.

It will be appreciated that, in a device as set out above, although on first passage of the beam through the irradiating zone there will only be a partial absorption of the radiation by the articles, on each subsequent passage of the beam through said zone there will be further absorption so that in total a greater proportion of the energy of the beam will be transferred to the articles than in devices hitherto known.

In recirculating the beam it is of course important to prevent the beam or any significant portion of it from returning to the souce, if possible damage to the latter is to be avoided. This may be done by coupling the other end of the wave-guide directly to the intermediate point and providing means to rotate the plane of polarisation of the microwave radiation through 90 during travel of the beam around the loop, a simple form of polarisation filter being interposed between the source and the loop. If a wave-guide of rectangular section is used, it is posice sible to rotate the polarisation plane by forming a length of the wave-guide with a twist. Alternatively, the rotation of the plane of polarisation may be done in the coupling between the other end and the intermediate point; most simply, said coupling may comprise a length of coaxial cable so connected to the other end and to the intermediate point of the wave-guide as to pick up the residual microwave energy when it first reaches said other end and return it to the wave-guide, at the intermediate point, polarised at 90 to the radiation from the source.

With the above relatively simple arrangements, however, residual microwave energy reaching the other end of the wave-guide for the second time is incorrectly polarised for passage through the coupling to the intermediate point and must be either absorbed or reflected. Absorption is wasteful, but if reflected then after travelling round the loop in the reverse direction some residual energy may reach the source and to keep this to a safe level the output of the source may need be restricted to a lower level than desirable.

At the expense of slightly greater complexity in the coupling between the other end of the wave-guide and the intermediate point, all microwave energy reaching the other end of the wave-guide may be returned to the intermediate point with its polarisation at 90 to that of energy directly received from the source, so that the latter is fully protected. This more complex but preferred arrangement involves the provision of two energy pickups adjacent to the other end of the wave-guide, one collecting energy polarised in the source plane and the other collecting energy polarised at 90 thereto, each pick-up being coupled by coaxial cable to a coupling unit, e.g. a directional coupler in which the energy received frm said two pick-ups is combined and delivered to two couplings at the intermediate point, both injecting energy into the wave-guide with polarisation at 90 to that of the source, the outputs to the two couplings being so phased as to cause the energy returned to the waveguide to be propagated away from the source.

Preferably the device includes means for automatic control of the output of the source by feed back of a control signal produced by sensing the intensity of the beam at some convenient part of the loop, so that the heating produced is substantially constant. Also, in some forms of device embodying the invention it is found that some small part of the beam returns to the source after repeated reflections, and such automatic control enables the source to be protected against undesirably high levels of returning radiation without unnecessary limitation of the beam intensity in the loop.

For a full understanding of the invention, preferred embodiments thereof will now be described with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a diagram of one form of device embodying the invention;

FIG. 2 is a section on line IIII of FIG. 1;

FIGS. 3 and 4 are views (similar to FIGS. 1 and 2 respectively) of a second form of device;

FIG. 5 is a view (similar to FIGS. 1 and 3) of a third form of device; and

FIGS. 6 and 7 are detail views of a modification applicable to any of the three forms of device shown in FIGS. 1 to 5.

Referring first to FIG. 1, the device there shown includes a microwave source 5 comprising a magnetron oscillator, delivering its output via a radiating antenna RA into a waveguide WG of rectangular section. Said waveguide is formed into a rectangular loop, having angled portions at three corners of the rectangle; one long side of the 3 rectangle is extended past the fourth corner, at which the adjacent short side overlaps the extended long side, making a T junction. The radiating antenna RA is located in the extended long side, outside the rectangular loop.

Between the antenna RA and the T junction forming one corner of the loop, a filter F1 is provided in the waveguide. A coupling device CD extends through the walls of the wave-guide where the two sides of the loop overlap at the T junction. Along the short side of the loop remote from the T junction a conveyor belt CB runs through the wave-guide in an enclosing tube ET of insulating material (see also FIG. 2). Said conveyor belt and its enclosing tube pass through openings in the wall of the wave-guide fitted with filter stubs FS.

In the long side of the rectangular loop remote from the T junction, the Wave-guide includes a twisted section TS. From this section to the T junction (going in the direction of arrows X) the smaller dimension of the wave-guide is measurable in the plane of FIG. 1, while the remainder of the wave-guide (including the extended long side of the loop and the short side through which the conveyor belt passes) is so orientated that its larger dimension is measurable in said plane.

Adjoining the twisted section TS, a further filter F2 is fitted to the wave-guide, and adjoining the filter F2 a sampling coupling SC is also provided; the sampling coupling, in the form of a power divider, extracts a small fraction of microwave energy passing through the waveguide and feeds it to a detector unit D which is adapted to produce an output signal, e.g. a DC. voltage, representing at any instant the intensity of the microwave beam then passing the coupling SC, and said output signal is carried by a control line CL to the source S in which said control signal is applied to control the output of the source in known manner so as to stabilise the intensity of the microwave beam in the wave-guide.

The coupling SC may be directional, i.e. may sample only the energy travelling in one selected direction, in which case the filter F2 is placed on the other side of coupling SC and the latter is arranged to sample returning radiation.

Turning to the operation of the device described, the microwave beam in the wave-guide travels as indicated by arrows X from the antenna RA past the filter F1 and the T junction, round the loop to the coupling device CD, and thence embarks upon a second journey round the loop. The filter F1 allows radiation from the antenna RA to pass, but reflects radiation polarised at 90 to the plane of polarisation of the beam from the antenna RA. Radiation from the coupling device CD is however polarised at 90 to said plane,-on account of the presence in the loop of wave-guide of the twisted section TS. Hence radiation from the coupling device CD cannot pass the filter F 1 and hence cannot return to the radiating antnenna IRA to damage the source S.

The filter F2 is similar to filter F1, but so orientated that radiation polarised at 90 to the aforesaid plane can pass it and hence serving to reflect radiation polarised in said plane.

The beam can, therefore, pass from the radiating antenna round the loop of wave-guide, start a second journey round the loop without feeding back towards the radiating antenna RA, and continue its second journey as far as the filter F2. Here, however, it is reflected, because on its second passage through the twisted section TS the beam has its plane of polarisation altered by a further 90, making 180 in all. The reflected radiation returns round the loop, undergoing a 90 polarisation change (in the opposite sense) as it returns through the twisted section; when it reaches the filter F1 the returning beam cannot pass, as its polarisation plane is still at 90 to that of radiation from antenna RA. In part, therefore, the beam will then oscillate backwards and forwards between the filters F1, F2. Part of the beam will however feed via coupling device 'CD to make a further return journey round the loop and this part of the beam then reaches the filter F1 so polarised that it is able to pass. However, the beam will necessarily by this time have undergone a number of successive attenuations and such part of the beam as returns to the radiating antenna can be kept to a safe level by suitable setting of the control means comprising detector D connected to source S.

The purpose of the device is to heat material (e.g. pellets of synthetic resin prior to their use in an injection moulding machine) and such material is fed through the device on conveyor belt CB. It will be apparent that in the part of the wave-guide WG through which the belt CB carries the material to be heated the mircrowave beam will be of several times the intensity of the beam generated by the source S and radiating antenna RA, due to the recirculation of the beam described above, hence for any given power delivered by source S, the heating of the material will be correspondingly faster than in known devices wherein no recirculation occurs.

The presence of material to be heated, and the absorption thereby of energy from the beam, causes a significant part of the attenuation of the beam on each of its journeys round the loop, and the attenuation due to this cause varies according to the quantity and nature of the material on belt CB. This attenuation is sensed by means of the sampling coupling SC and detector D, and the control of source S in accordance with this sensing causes the output of source S to be increased to balance this attenuation, the control of such increase being however so arranged that the proportion of the radiation which (as explained above) ultimately returns to the radiating antenna is never permitted to reach a level at which damage to the source is possible. The flow of material carried by conveyor belt CB may be allowed to vary, as the consequent variations in microwave beam attenuation are automatically balanced (by the stabilising control exercised through detector D) thus enabling the fastest practicable heating to be obtained in all circumstances.

Between the sampling coupling SC and the coupling device CD a phasing network PN is provided, to permit adjustment of the electrical length of the loop of waveguide.

Turning now to FIG. 3, various parts of the device here shown are generally similar to those of FIGS. 1 and 2 and hence will not be described again, but merely given the same references on the drawing.

The principal difference in the device of FIG. 3 is that the wave-guide WG has no twisted section TS, and is not formed into a complete loop but into a substantially U shape. The end portion of the wave-guide remote from the microwave source S is linked to the coupling device CD by coaxial cable CC, having a pick-up coupling PC in said end-portion. The sampling coupling SC is placed in the cable CC, which cable also contains a phasing network PN, and the coupling PC and coupling device CD are arranged respectively to pick up radiation with the polarisation of source S and to inject energy into the wave-guide with polarisation at to that of the source S.

The operation of the device of FIG. 3 is similar to that of the device of FIG. 1; it will however be noted that as the change of polarisation plane is effected by the relationship between the couplings at the two ends of the coaxial cable CC and the wave-guide WG, and hence no twisted section TS is needed, equally it is not necessary to employ a rectangular-section wave-guide and (as shown in FIG. 4) the wave-guide WG of FIG. 3 is of circular section.

The device of FIG. 5 again has much in common with the devices of FIGS. 1 and 3 and the same references are given to similar parts. As in FIG. 3, the loop round which microwave radiation circulates is in part made 'of coaxial cable. In the device of FIG. 5, however, a rather more complex cable connection is provided. In the end portion of wave-guide WG remote from source S, there are two pick-up couplings -PC1 and PC2, connected by coaxial cables CCl, CCZ respectively to a directional coupler unit DCU. From the latter unit two coaxial cables CCS, CO4 deliver microwave power to two coupling devices CD1, CD2 respectively for recirculation, the cables CC3 and CC4 containing phasing and/or attenuation networks PANl, PAN2 respectively. Filter F2 is in this case placed between the coupling PC1 and the coupling PC2.

The pick-up coupling P02 is arranged to collect radiation polarised in the plane of radiation direct from source S (i.e. on its first passage round the loop) while pick-up coupling PC1 collects radiation polarised at 90 to that direct from the source (i.e. on its second or subsequent passage). Coupling devices CD1, CD2 are both arranged so that radiation they inject into the wave-guide WG is polarised at 90 to that coming direct from the source through filter F1; also, the phases and amplitudes of the energy so injected by devices CD1, CD2 are so regulated by networks PANl, PANZ and the unit DCU that the radiation from said devices propagates forward (i.e. as indicated by arrows X).

The networks PANl, PANZ are desirably adjustable to provide correct phasing and amplitude at each of the devices CD1, CD2. A convenient arrangement is for one of said networks to serve as an attenuator and the other as a phase-shifter; the latter may with advantage be arranged to be controlled automatically by the power levels applied to the two inputs of the directional coupler unit DCU, as it is the ratio between these levels that determines the relative phasing of the two outputs from the coupler unit. 1

Sampling coupling SC is in the device of FIG. 5 placed at a point between the coupling devices CD1, CD2 and the part of the wave-guide WG through which the conveyor belt CB passes; said sampling coupling is arranged to extract a fraction of the total microwave beam passing it, i.e. to sample both the radiation direct from source S and that recirculated with different polarisation.

In relation to all three forms of device described above, it will be understood that the drawings are diagrammatic and that the form and dimensions of the various parts will be determined in any particular case by conventional methods. For example, the filters F1, F2 are represented as simple dashed lines but may each comprise a plurality of parallel screens, each screen comprising parallel wires, said wires and screens being at carefully set distances from their neighbours. The closed ends of the wave-guide WG of each form of device will normally have adjustable caps to allow tuning of the end portions of the waveguide, so that the antenna RA (FIGS. 1, 3 and 5) and pick-up couplings PC (FIG. 3) and PC1 (FIG. 5) may be correctly spaced from said closed ends, and filter stubs PS (FIGS. 1, 3 and 5) may be adjustable also for tuning purposes.

A known characteristic of dielectric heating by highfrequency electromagnetic radiation is that the heat is generated within the material being heated. When pellets, for example, are to be heated by this method, their interior may reach a maximum desired temperature while their outer parts are still at a lower temperature than desired. In the devices described, therefore, there may be included provision for heating the exterior of material travelling on the belt CB. While it may be satisfactory simply to provide means such as radiant heaters adjoining the path of belt CB outside the wave-guide WG, more conveniently radiant heating may be applied simultaneously with dielectric heating by making the section of the wave-guide through which the material passes, or the section of tube ET within the wave-guide, of electrically conductive material of such resistance as to absorb part of the microwave radiation, so that the temperature of that section of wave-guide or tube is raised and the wave-guide or tube itself functions as a radiant heater. It will be seen that the radiant heating thus applied is necessarily kept in a fixed ratio to the dielectric heating. If it is necessary to provide for variation of this ratio, this may be done by providing adjustable cooling by any convenient means.

Where no radiant heating is desired, it may be found desirable to provide for cooling of the wave-guide even when the material of the latter is of low resistance.

Lastly, although in all three forms of device shown a conveyor belt CB is incorporated to carry material for heating through the device, FIGS. 6 and 7 illustrate a modification particularly adapted for use when pellets e.g. of synthetic resin, are to be heated.

FIGS. 6 and 7 show a part of the wave-guide WG of any one of the devices of FIGS.l-5, namely that part which, in said earlier Figures, accommodates the conveyor belt CB. As best seen in FIG. 6, the conveyor belt and its enclosing tube are omitted, hence removing the need for the filter stubs FS also. This part of wave-guide WG as illustrated in FIG. 6 has a gap occupied by a peripheral portion of a rotatable disc RD. The disc RD has a carrier insert CI, of material not significantly heated when subjected to microwave radiation (e.g. of ceramic or polytetrafluorethylene) and said insert has four carrier pockets CP each containing a pellet P of material to be heated. Elsewhere on the disc RD further similar inserts (not shown) may be provided, so that succesive groups of pellets may be subjected to the microwave radiation by successive partial rotations of the disc RD.

What I claim as my invention and desire to secure by Letters Patent is:

1. A heating device comprising a wave-guide a source of microwave energy coupled to one end of said waveguide to radiate said energy as a beam along a path defined by said wave-guide, means connecting the other end of said wave-guide to an intermediate point thereon whereby said connecting means and a part of said waveguide form a closed loop for the transmission of said energy, said connecting means being such that any of said energy reaching said other end passes through the connecting means and is radiated from said connecting means into said wave-guide to travel in a direction away from said source, means for rotating the plane of polarization of the energy through during travel around the loop consisting of said connecting means and said part of said wave-guide whereby said energy radiated by said connecting means into the wave-guide has its plane of polarization at right-angles to the plane of polarization energy entering the loop from the source, a polarization filter interposed between said source and said intermediate point to permit microwave energy from said source to pass therethrough and to reflect microwave energy having its plane of polarization at right angles to that of energy from said source, an irradiating zone comprising a portion of the path of said beam within said wave-guide, and means for positioning articles to be heated in said irradiating zone.

2. A device as claimed in claim 1, in which the waveguide is of rectangular section and said means for rotating the plane of polarization comprises a length of the wave-guide within the loop which is formed with a 90 twist in the direction of its length.

3. A device as claimed in claim 2 comprising a further polarization filter interposed between said length of waveguide formed with a 90 twist and said connecting means to permit microwave energy from said source to pass therethrough and to reflect microwave energy having its plane of polarization at right-angles to that of energy from said source.

4. A device as claimed in claim 1, in which said means for rotating the plane of polarization is incorporated in said connecting means so arranged that energy passing therethrough is radiated into the wave-guide with its plane of polarization at 90 to that of energy radiated from said source.

5. A device as claimed in claim 4 comprising a further polarization filter interposed between said intermediate point of said loop and said connecting means to permit microwave energy from said source to pass therethrough and to refiect microwave energy having its plane of polarization at right-angles to that of energy from said source.

6. A device as claimed in claim 1, in which said connecting means comprises a length of coaxial cable.

7. A device as claimed in claim 1 wherein said connecting means comprises two energy pick-ups adjacent to the other end of the wave-guide, one of said pick-ups being arranged to collect energy polarized in the source plane and the other of said pick-ups being arranged to collect energy polarized at 90 to the source plane, a coupling unit, coaxial cable connecting said pick-ups to said coupling unit, said coupling unit having two outputs and means connecting said outputs to said intermediate point of said wave-guide, said coupling unit being so arranged that the phasing of the two outputs is such as to cause the energy returned to the wave-guide to be propagated away from the source.

8. A device as claimed in claim 1, including means for sensing the intensity of the beam in the loop and producing a control signal, and means for feeding back said control signal to the source to control the output thereof.

9. A device as claimed in claim 7 comprising a further polarization filter interposed in said wave-guide between said two energy pick-ups to permit microwave energy from said source to pass therethrough and to reflect microwave energy having its plane of polarization at right- 25 angles to that of energy from said source.

10. A device as claimed in claim 1 wherein said means for positioning articles to be heated in said irradiating 53 zone comprises means for conveying articles through said irradiating zone.

11. A device as claimed in claim 1 comprising a further polarization filter in said wave-guide on the opposite side of said irradiating zone from said source to permit microwave energy from said source to pass therethrough and to reflect microwave energy having its plane of polarization at right-angles to that of energy from said source.

References Cited UNITED STATES PATENTS 2,495,429 1/1950 Spencer 21910.55 2,790,054 4/1957 Haagensen 2l910.55 3,210,513 9/1965 Lenart 21910.55 3,281,567 9/1966 Meissner et a1.-- 2l9-10.77 X 3,286,208 11/1966 Niebuhr et a1. 219-1055 X 3,344,363 9/1967 Console et a1.-- 21910.55 X 3,412,227 11/1968 Anderson 21910.55

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner US. Cl. X.R. 33383 

